Polyurethane foams with a wide variety of physical properties have long been manufactured on a commercial scale by the known isocyanate polyaddition process from compounds containing several active hydrogen atoms, in particular compounds containing hydroxyl and/or carboxyl groups, and polyisocyanates with the addition of water and/or blowing agents and optionally catalysts, emulsifiers and other additives (Angew. Chem. A, 59 (1948), page 257). Given a suitable choice of components, either flexible or rigid foams or any products between these extremes may be obtained.
Polyurethane foams are preferably produced from liquid starting components, either by mixing all the starting materials together in a one-shot process or by first preparing a prepolymer containing isocyanate groups from a polyol and an excess of polyisocyanate and then foaming this prepolymer, e.g. by reaction with water.
Tertiary amines have become well established as catalysts for the production of polyurethane foams. They accelerate the reaction of hydroxyl and carboxyl groups with isocyanate groups (urethane reaction) and the reaction between water and isocyanates (blowing reaction). The velocities of the two reactions which take place simultaneously in the one-shot process have to be adjusted relative to each other.
Cross-linking reactions which give rise to the formation of allophanate, biuret and cyanurate structures take place during the foaming process in addition to the reactions mentioned above.
In view of the large number of reactions taking place, the catalyst must be chosen so that it ensures synchronous adjustment of the reactions to each other. At the same time the catalyst must not be fixed too early in the process by incorporation in the foam, nor must it subsequently accelerate hydrolytic degradation of the foam product. This problem has not up to now been completely solved. The unpleasant smell of many of the tertiary amines used in practice is another disadvantage. Moreover, polyurethane foams, both alone and laminated with colored synthetic resin foils (e.g. PVC foils) tend to undergo yellowing or discoloration and blackening on exposure to heat or light in the presence of the industrially used amine catalysts, such as "Dabco" or bis-(dialkylaminoalkyl)-ethers (see, e.g. German Offenlegungsschrift No. 1,804,361 and U.S. Pat. No. 3,330,782. Blackening of colored synthetic resin foils of the kind used commercially for cladding polyurethane foams, e.g. in motor car seats, refrigerator bodies and electrical instruments, is particularly troublesome. These disadvantageous effects prevent polyurethane foams and polyurethane synthetic resins from being used in fields of application for which they would otherwise be suitable.
Catalysts have now surprisingly been found which, used alone or as mixtures with known amine catalysts, prevent the discoloration of foam-laminated synthetic resin foils (e.g. PVC foils) on exposure to heat and/or light. These catalysts also prevent the effects of ageing in polyurethane synthetic resins which have been foamed in unrestricted space.